Coaxial line reflectometer having a resistance connected between sections of the outer conductor



March 29, H F JARGER ETAL COAXIAL LINE REFLEGTOMETER HAVING A RESISTANCECONNECTED BETWEEN SECTION OF THE OUTER counucwon Filed y 3. 1962 sSheets-Sheet 1 wig- IND/CA TING MID/CAT/IVG 05 V/C6 DE VICE REE IIVC.

INVENTORS. HAROLD F, JARGER R OBEKT 4L/S ROBERT 7. ADA/7S ATTORNEY March1965 H. F. JARGER ETAL 3243'704 COAXIAL LINE REFLECTOMETER HAVING ARESISTANCE CONNECTED BETWEEN SECTION OF THE OUTER CONDUCTOR Filed July23. 1962 5 Sheets-Sheet 2 INVENTORS. HAROLD F. JARGER y ROBfRT LL/SROBERT 7: A DAMS ATTORNEY March 29. 1966 H. F. JARGER ETAL 3243704COAXIAL LINE REFLECTOMETER HAVING A RESISTANCE CONNECTED BETWEEN SECTIONOF THE OUTER CONDUCTOR Filed July 23, 1962 5 Sheets-Sheet 5 l C 1 LINVENTORS. HAROLO F. JARGER By ROBERT ELL/S ROBERT 7: AOAMS A 7' TURNEYUnited States Patent son Heights, N.Y., and Robert T. Adams, ShortHills,

NJ., assignors to International Telephone and Telegraph Corporation,Nutley, NJ., a corporation of Maryland Filed July 23, 1962, Ser. No.211,776 7 Claims. (Cl. 324-95) This invention relates generally tocoaxial transmission line directional couplers and particularly tobidirectional couplers commonly termed reflectometers.

As disclosed on page 896 of Techniques of Microwave Measurements, volume11, MIT Laboratory series edited by C. G. Montgomery, a reflectometernormally comprises two directional couplers which are insertable into atest transmission lineone to measure incident RF. power and the other tomeasure reflected RF. power at the point of insertionto provide ameasure of the reflection coefiicient, or the voltage standing waveratio, of the line.

Prior art coaxial transmission line directional couplers generallyemploy a bulky arrangement involving either a resistor in series withthe center conductor of the coaxial line with appropriate leads broughtout through high voltage insulated openings in the outer conductor ofthe coaxial line, or a chamber in which is placed a transformer typecoupling which extracts the energy to be measured. These structures aregenerally inadequate because of the aforementioned bulkiness and alsobecause of the necessity for complex high voltage insulating structuresor complex current transforming structures, or other designcomplications.

These and other disadvantages found in prior art apparatus are avoidedby means of the present inventive arrangement wherein a relatively lowvoltage, with respect to the outer coaxial conductor, is developedacross a resistor placed in series between separated portions of theouter conductor, the thus developed low voltage being compared to thevoltage developed across a capacitive voltage divider network runningbetween the center conductor and one of the separated outer conductorportions. This arrangement has been found to result in an overallstructure of considerably reduced size, of convenient accessibility foradjustment of the strength of the signal extracted from the main line,and also of improved performance characteristics.

A problem arising with this type of arrangement, however, is that,because of its position in series with the outer conductor, it isdiliicult to enclose the above-mentioned series resistor and outerconductor portions in a metal housing, for shielding and packagingpurposes, without, in effect, shorting out the signal to be measured.This problem has been solved by providing a ferrite core structurebetween the outer housing shield of the apparatus and the outerconductor of the coaxial structure, to which the series resistance isconnected, so that a relatively high impedance is presented to energypropagating between the outer surface of the outer conductor and thehousing shield.

It is therefore a general object of this invention to provide animproved directional coupler apparatus.

A more specific object is to provide an improved coaxial linerefiectometer.

A still more specific object is to provide a coaxial line reflectometerin which signals coupled out of a coaxial line are maintained at a lowvoltage with respect to the outer conductor of the coaxial line.

Another specific object is to provide coaxial line reflectometerapparatus wherein signals are extracted from a coaxial line at arelatively low voltage with respect to the outer conductor of the line,by means of a relatively small resistance which is connected in seriesbetween separated portions of the said outer conductor, and which iscontained within a metal shielding housing which is isolated fromsignals developed across the said series resistance.

These and other objects and features of the present invention may bemore fully understood and appreciated from the following detaileddescription of exemplary reflectometer apparatus designed to accommodateone kilowatt of energy in the main coaxial line while dissipating onlyabout 10 watts of energy in the VSWR monitoring structure, saiddescription to be read in association with the accompanying drawingwherein:

FIGURE 1 is a schematic drawing illustrating the electrical circuitconfiguration corresponding to the reflectometer structure of thepresent invention,

FIGURE 2 is a plan view illustrating the actual physical structure ofthe present refiectometer apparatus,

FIGURE 3 is a sectional view taken through the axis of the coaxialstructure within the reflectometer apparatus illustrated in FIGURE 2,

FIGURE 4 is a view of a section through the apparatus of FIGURE 3 takenalong the lines 44 transverse to the axis of the coaxial structure shownin FIGURE 2, and

FIGURES 5 and 6 are partial schematics for explain: ing the theory ofoperation of the present apparatus.

Referring to FIGURES 1 through 4, exemplary reflectometer apparatus,having a fiat output response within the frequency range 2-30megacycles, and constructed in accordance with the teachings of thisinvention, comprises a metal shield and housing 1 having coaxialconnectors 2 and 3 connected thereto for respectively making output andinput terminal connections to separated sections 4 and 4a, of a coaxialline, so that RF. energy propagating down the line 4 enters the housingat 2, passes through apparatus to be described within the housing,emerges from the housing at 3 and continues down the coaxial linesection 4a to a load termination 5, at which the energy is either fullyabsorbed or partially reflected back towards the terminal 3 through thehousing, out of the terminal 2 and back through the line section 4. Thefunction of the refiectorneter apparatus within the housing 1 is toprovide minimal interference with the above mentioned energy propagationwhile utilizing a portion of the propagating energy to produce a signalor signals representative of the VSWR (voltage standing Wave ratio) atthe point of insertion of the apparatus in the line 4, 4a, thedimensions of the housing 1 being small in relation to a quarterwavelength of the propagating signal.

A center conductor 6 extends along the axis of the connector 2 andthrough the housing 1 to the connector 3. The corresponding outerconductor of the coaxial structure comprises the outer portion 2a of theconductor 2, a hollow conductive section 7a connected thereto, acylindrically arranged resistance indicated generally at 8, in serieswith the section 7a, and a hollow conductive section 7b connected to theresistance 8, and to the outer portion 3a of the connector 3. Toroidalferro-electric cores 9 and 10 are placed in corresponding grooveslocated between the outer surfaces of the outer conductor sections 70and 7b and the respective connections between the connectors 2 and 3,and the housing 1 to isolate RF. signals propagating along the outersurfaces of the sections 7a and 7b from the grounded shield 1, forpurposes discussed hereinafter, it being noted at this point that RF.energy entering the reflectorneter via either connector 2 or connector3-will be confined to the interior of the coaxial structure until itreaches the resistor 8 at which point some of the energy may leak out ofthe coaxial enclosure and propagate along the outer surface of section70 or section 7/) to the inner surface of the housing 1, therebybypassing resistance 8. It is this outer propagation which is impeded bythe ferroelectric cores 9 and 10, which, in effect, act as RF. chokes.The impedance offered to currents flowing on the outer surfaces of thecoaxial connector is increased by the placing of the ferrite cores. Thisportion of the circuit can be represented as a toroidal inductor of oneturn. The inductive reactance of a torroid can be expressed as:

where A =cross sectional area of core l =average flux path length N=number of turns =permeability of free space n =permeability of core Kconstant of proportionality In this device N=l, A /1 -.5, but a /n maybe in the order of several hundred to a thousand, resulting in a markedincrease in inductive reactance.

On either side of the resistance 8, the coaxial structure containshollow tubular conductive members 11 and 12 which provide apredetermined amount of distributed capacitance extending from thecenter conductor 6 to the tubular members, and also extending from thesaid respective tubular members to the respective outer conductorsections 7a and 7b. The distributed capacitances between the centerconductor 6 and the tubular members 11 and 12 are designatedrespectively 11a and 12b while those between the said members 11 and 12and the corresponding outer conductor sections 7a and 7b, arerespectively designated 11b and 12b.

Conductive connections 13 and 14 are brought out through respectiveopenings 15 and 16, in the outer conductor sections 7a and 7bwhichopenings are suitably insulated by means of dielectric spacers 15a and16a, respectively-from the respective tubular members 11 and 12, torespective terminals 17 and 18. Fixed and variable capacitors, 19 and20, respectively, are connected in parallel between terminal 17 andouter conductor section 7a, while a symmetrically disposed pair of fixedand variable capacitors, 21 and 22, respectively, are connected inparallel between terminal 18 and section 7b. The interior of the coaxialstructure is filled with an insulating dielectric such as polyethyleneor Teflon, indicated generally at 23, in which the tubular metal membersare embedded.

The terminals 17, 18 and components 19-22 are all mounted on a terminalboard indicated generally at 24. From FIGURE 1 it is clear that thecapacitors 19 and 20 are in parallel with distributed capacitance 11b,and likewise that capacitors 21 and 22 are in parallel with distributedcapacitance 12b.

An incident RF. signal detection network comprises the seriescombination of a conductor LES-which is connected to outer conductorsection 7a at one end of resistance 8a crystal rectifier 26, terminal 8,a resistor 27, and an output terminal 28 all mounted on board 24. Abypass capacitor 29 is connected between the ground of housing 1 or anyother suitable ground, and terminal 28.

Similarly, a reflected RF. signal detection network comprises the seriescombination of a conductor 30 which is connected to the outer conductorsection 7b at the opposite end of resistance 8 relative to conductor 25,a crystal rectifier 31, terminal 17, resistor 32, and output terminal33, all mounted on board 24, with a bypass capacitor 34 connectedbetween terminal 33 and the housing ground.

It will be shown that the series combination of capacitance 11a, and theparallel capacitances 11b, 19 and 2 represents an adjustable voltagedivider which transfers a signal proportional to the incident RF. signalappearing across coaxial conductor s 6 and 7a to terminal 17, andsimilarly, that capacitance 12a in series with the parallel capacitances12b, 21, and 22, represents an adjustable voltage divider for reflectedsignals with respect to terminal 18.

The theory of operation is as follows:

This device makes use of the phenomenon relating to the existence of acurrent, on the inner surface of the outer conductor of a coaxialtransmission line, which is equal and opposite to the current flowing inthe center conductor. This current on the outer conductor produces avoltage E (FIGS. 5, 6) across R(8) (FIGS. 5, 6) which is proportional tothe current 1;, on the line.

The voltage and current on the transmission line can be expressed as:

L i-i-r IL: o i r) where The voltage across the resistor R can beexpressed as:

E (EL) If the polarities of the voltages due to an RF. source are asshown in FIG. 5, then it can be seen that V is the difference voltagebetween E and E If E is adjusted to be equal to E when E is equal tozero, as is the case when the line is terminated in Z then for any othervalue of Z V becomes:

whence V is a function of the reflected voltage on the line and will beequal to zero for a properly terminated transmission line. When usedwith a suitable metering circuit, such as that indicated at 41 in FIG.1, the output will be an indication of reflected power.

Referring to FIG. 6 the voltage and current on the line are, aspreviously mentioned:

The voltage E across the voltage divider can be expressed as:

It can be seen in FIG. 6, that V is equal to the sum of B and E If E isadjusted to be equal to E when E,- is equal to zero, i.e. when thetransmission line is terminated in Z then for any other value of whenceV is a function of the incident voltage and with a suitable meteringcircuit will indicate incident power. Thus, by adjusting capacitances2t) and 22, the signals at terminals 28 and 33, taken with respect toground, are,

respectively proportional to the incident and reflected signal levels.These signals may then be monitored by suitably calibrated D.C.microammeters, as shown at 40 and 41, respectively.

A table of specific dimensions and component values used in theconstruction of the above discussed reflectometer structure is given intable 1 below:

TABLE 1 Description Male connectors, LC type UG287/U.

Metal elbow joints, LO type UG208/U.

Ferroelectric cores, 1% I.D., 2 0. length.

Tubular capacitance plates approximately 1 inch in length by inch inmean diameter, by inch thick in parallel between sections 7a and 7b toprovide a total resistance of 0.5 ohms.

It is noted that while for the present purposes it has been foundexpedient to construct the resistance 8 out of a plurality of lumpedresistors cylindrically disposed, those skilled in the art willappreciate that under certain circumstances a deposited film resistormight provide an even more effective structure, and that such depositionof a resistor is easily within the skill of those familiar with thisart. Accordingly, the construction indicated above is not to beconstrued as a limitation on this invention.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of our invention as set forth in the objects thereof and inthe accompanying claims.

We claim:

1. Coaxial transmission line directional coupler apparatus comprising:

(a) A first hollow outer conductor section,

(b) A second hollow outer conductor section,

(0) A central conductor coaxially disposed within said first and secondsections,

((1) A resistance connected in series electrical circuit between saidfirst and second sections,

(e) A first voltage divider network connected between said centralconductor and said first section,

(f) A second voltage divider network connected between said centralconductor and said second section,

(g) A first signal detection network, coupled between said secondsection and said first voltage divider network,

(h) A second signal detection network coupled between said first sectionand said second voltage divider network,

(i) And means coupled to said first and second signal detection networksfor monitoring output signals therefrom.

2. A coaxial line reflectometer comprising:

(a) A first hollow outer conductor section,

(b) A second hollow outer section axially aligned with said firstsection,

(c) A central conductor coaxially disposed within said first and secondouter sections,

(d) A coupling resistance connected between said first and secondsections,

(e) A first voltage divider network connected between said centralconductor and said first section,

(f) A second voltage divider network connected between said centralconductor and said second section,

(g) A first signal detection network connected between said secondsection and said first voltage divider network,

(h) A second signal detection network connected between said firstsection and said second voltage divider network,

(i) And means coupled to said first and second signal detection networksfor jointly monitoring the outputs thereof.

3. A coaxial line directional coupler comprising:

(a) A first hollow outer conductor section,

(b) A second hollow outer conductor section axially aligned with saidfirst section,

(c) A central conductor coaxially disposed within said first and secondouter sections,

(d) A coupling resistance connecting said first and second sections,

(e) Capacitive voltage divider means connected between said firstsection and said central conductor, (f) Signal detection means coupledbetween a tap point on said capacitive voltage division means and saidsecond section,

(g) A metal housing shield surrounding said coupling resistance,

(h) And means interposed between the end of said coupling resistanceconnected to said first section, and the said metal housing forpreventing leakage of RF. currents between the outer surface of saidfirst section and the inner surface of the said housing.

-4. Coaxial line reflectorneter apparatus comprising:

(a) A first hollow outer conductor section,

(b) A second hollow outer conductor section coaxially aligned with saidfirst section,

(c) A cylindrically disposed hollow resistance connected between saidfirst and second sections,

((1) A central conductor coaxially disposed within the enclosure definedby said first section, said connected resistance, and said secondsection,

(e) A capacitive voltage divider network connected between said centralconductor and said first section, including a hollow conductive tubularmember disposed coaxially intermediate said first section and saidcentral conductor, which tubular member provides a predetermineddistributed capacitance between the said central conductor and the saidmember and also a predetermined distributed capacitance between the saidmember and the said first section,

(f) A signal detection network connected between the said second sectionand the said tubular conductive member,

(g) Means connected between said tubular conductive member of said firstsection for varying the voltage division ratio associated with saidvoltage divider network,

(h) And means coupled to said tubular member for a monitoring thesignals appearing thereat.

5. Coaxial line reflectometer apparatus comprising:

(a) A central conductor,

(b) A first hollow outer conductor section coaxially surrounding saidcentral conductor,

(c) A second hollow outer conductor section axially aligned with saidfirst section, and coaxially surrounding said central conductor,

(d) A resistance disposed in hollow tubular fashion around said centralconductor, and electrically connecting said first and second sections,

(e) A first tubular shaped hollow conductive member coaxially disposedintermediate said central conductor and said first section,

(f) A second tubular shaped hollow conductive member coaxially disposedintermediate said central conductor and said second section,

(g) A first signal detection network coupled between said second sectionand said first tubular member,

(h) A second signal detection network coupled between 7 said firstsection and said second tubular member,

(i) Means coupled between said first tubular member and said firstsection for varying the impedance presented by said tubular member toincident R.F. energy propagating in said first section,

(j) Means coupled between said second tubular member and said secondsection for varying the impedance of said second tubular member toreflected RF. energy propagating in said second section,

(k) A conductive housing enclosing all of the abovernentioned items andhaving electrical connections to the outer surfaces of said first andsecond sections at given regions thereof,

(1) And means intermediate said given connection regions to said firstand second sections and said resistance for impeding leakage of RF.currents between the exterior of said hollow resistance and saidconductive housing.

6. Apparatus according to claim 5 wherein:

(a) The outer surfaces of said first and second sections are inwardlygrooved between said connecting region and said resistance,

(b) And said leakage impeding means includes first and secondring-shaped ferroelectric core members respectively mounted in saidinput and output sheath section grooves.

7. Coaxial transmission line reflectorneter apparatus comprising:

(a) A first hollow outer conductor section,

(b) A second hollow outer conductor section having an axis in line withan axis of said first section, (c) A cylindrically disposed resistanceconnected between said first and second sections,

(d) A central conductor threading said first and second sections andsaid resistance,

(e) First and second capacitive voltage divider networks connectedbetween said central conductor and said respective first and secondsections,

(f) First and second RF. signal detection means coupled between saidrespective first and second Voltage divider networks and said respectivesecond and first sections,

(g) First and second voltage adjustment means connected to saidrespective first and second voltage divider means for independentlyadjusting the respective outputs thereof to be equal to the voltage dropacross said resistance, when the coaxial line containing said sections,said resistance, and said central conductor, is terminated in itscharacteristic impedance, over a wide band of input frequencies,

(h) and first and second indicating means respectively coupled to saidfirst and second signal detection means for indicating the levels ofrespective reflected and incident signals passing thru said resistance.

References Cited by the Examiner UNITED STATES PATENTS 2,423,447 7/1947Grimm 324- 2,588,390 3/1952 Jones 324-95 2,925,565 2/1960 Cook 33324.l

WALTER L. CARLSON, Primary Examiner.

R. V. ROLINEC, Assistant Examiner.

1. COAXIAL TRANSMISSION LINE DIRECTIONAL COUPLER APPARATUS COMPRISING:(A) A FIRST HOLLOW OUTER CONDUCTOR SECTION, (B) A SECOND HOLLOW OUTERCONDUCTOR SECTION, (C) A CENTRAL CONDUCTOR COAXIALLY DISPOSED WITHINSAID FIRST AND SECOND SECTIONS, (D) A RESISTANCE CONNECTED IN SERIESELECTRICAL CIRCUIT BETWEEN SAID FIRST AND SECOND SECTIONS, (E) A FIRSTVOLTAGE DIVIDER NETWORK CONNECTED BETWEEN SAID CENTRAL CONDUCTOR ANDSAID FIRST SECTION, (F) A SECOND VOLTAGE DIVIDER NETWORK CONNECTEDBETWEEN SAID CENTRAL CONDUCTOR AND SAID SECOND SECTION, (G) A FIRSTSIGNAL DETECTION NETWORK, COUPLED BETWEEN SAID SECOND SECTION AND SAIDFIRST VOLTAGE DIVIDER NETWORK, (H) A SECOND SIGNAL DETECTION NETWORKCOUPLED BETWEEN SAID FIRST SECTION AND SAID SECOND VOLTAGE DIVIDERNETWORK, (I) AND MEANS COUPLED TO SAID FIRST AND SECOND SIGNAL DETECTIONNETWORKS FOR MONITORING OUTPUT SIGNALS THEREFROM.